This invention relates to a control for controlling rotation as between a pair of driven shafts driven by a common drive shaft.
An application of the present invention is in the differential of a vehicle""s drive line. A propeller shaft provides the drive power and is connected to the differential. Aligned axles extend from the differential at a direction perpendicular to the propeller shaft. An arrangement of gears in the differential transmits torque from the propeller shaft to the axles which in turn transmit the torque to a pair of wheels. The torque of the axles is always equal regardless of the speed of the axles relative to each other. When the axles are connected to wheels having similar tractive capacity, the axles rotate equally or, if the vehicle is in a turn, then they rotate differently according to the turning radius of each wheel. Differential axle rotation in this case is desirable for normal vehicle operation. When the axles are connected to wheels having substantially different tractive capacity, the wheel having lesser tractive capacity may slip, thus causing the axle connected to it to turn faster than the axle connected to the wheel having greater tractive capacity. Differential axle rotation in this case is undesirable for normal vehicle operation.
The invention in its preferred form utilizes a planetary gear arrangement. FIG. 1 is a schematic view of such a planetary gear arrangement. Note that shafts 10 and 12 are connected to two axle shafts 50 and 52 through auxiliary gearing (20a, 20b, 22a, 22b) and that shaft 10 is connected to ring gear 14 and shaft 12 is connected to the axis of the planet gears 16. It will be appreciated that reference 18 indicates the sun gear. The rotation of shafts 10 and 12 can be controlled via the coupling of the differently sized auxiliary gears 20a, 20b and 22a, 22b so that sun gear 18 does not rotate as long as the axle shafts 50, 52 rotate at the same speed.
In the example illustrated, ring gear 14 has to rotate faster than the axes of planet gear 16 so that the gear teeth of the planets merely walk around the sun gear. This relationship can be calculated and through various gear reduction technologies, the axles can generate the relative rotation of the shafts 10 and 12 when the wheels or axles are rotating at the same speed to produce zero rotation of sun gear 18. With this relationship established, if the wheel axles are rotated at anything other than equal speed, the relative rotation between shafts 10 and 12 will change and sun gear 18 is then rotated.
As previously explained, it is desirable to allow a difference in rotation as between the wheel axles when turning but not desirable when one of the wheels is slipping. The difference in rotation between the wheels when the vehicle is turning is much less than what wheel slippage will generate. One type of control that responds to the difference in rotation is a centrifugal clutch. FIG. 2 is also schematic and illustrates a centrifugal clutch mechanism for the system of FIG. 1. Sun gear 18 having shoe members 24 are spring biased by springs 26 toward the sun gear 18 and away from a fixed ring 28. When sun gear 18 is rotated, centrifugal force will urge outward movement of the shoes 24 until springs 26 are overpowered whereupon the shoes 24 will engage the fixed ring 28. The shoes 24 then become brake shoes and tend to prevent the shaft 18 from turning any faster. Yet depending on the spring force 26 and shoe weight, some rotation of shaft 18 is permitted before braking will be engaged and this can be designed to accommodate the desired difference for vehicle turning while preventing significant slipping.
Whereas the above control is basically one which allows a determined differential rotation and prevents anything beyond that rotation, there is a need for a more flexible control of the braking arrangement, e.g., the ability to sense different situations, the ability to more rapidly respond, etc., which will hereafter be referred to sometimes as xe2x80x9csmartxe2x80x9d control. Thus, a preferred embodiment of the invention having smart control incorporates a Magnetorheological Fluid (MRF) clutch and an electronic controller which is schematically illustrated in FIG. 3.
With reference to FIG. 3, which illustrates an alternative xe2x80x9cbrakexe2x80x9d to that of FIG. 2, the sun gear 18 is surrounded by fixed ring 28xe2x80x2. Interleaved plates, extending inwardly from ring 28xe2x80x2 (plate 30) and outwardly from sun gear 18 (plates 32) are spaced closely together. The spacing between the plates is filled with MRF (indicated by reference 34). The dot-dash lines 36 passing through the plates 30, 32 represent a magnetic field generated by an electromagnetic coil 42. The electromagnetic coil is activated by electronic controller 38. A rotary permanent magnet 40 is provided on the sun gear 18 and generates an impulse that is detected by a Hall Effect Device incorporated into the controller 38.
MRF is a fluid that carries ferrous particles and when no magnetic field is applied, the fluid, which has a low viscosity, generates little or no resistance to relative movement of the plates 30, 32 and thereby permits free relative rotation of the sun gear 18. When a magnetic field is applied, the particles become polarized and assume a very different property which can be best explained as having a high apparent viscosity. The effect of the higher apparent viscosity material is that of a brake that resists rotation of plates 32 relative to plate 30. The electronic controller 38 generates a magnetic field in response to a set of programmable instructions which in turn is responsive to the impulse generated by permanent magnet 40.
The electronic controller 38 thus monitors the rotative action of the magnet 40 and may react to rotative speed thereof (e.g., the magnetic coil is energized when sun gear 18 reaches a given rpm) or it may react to the angular acceleration of magnet 40 (i.e., the rate of change of the rate of rotation of sun gear 18). When one wheel engages a slippery surface, that wheel rapidly accelerates which can be detected by the controller.
The program may be established to trigger a linear actuation which will produce a lower viscosity and sluggish braking rather than abrupt braking. The program may be altered at will through a manual control provided to the driver who can thus change the permitted rotation of the sun gear from zero rotation to substantially unlimited rotation. Such adjustability is particularly desirable for an all terrain vehicle but, of course, is not limited to such a vehicle.
A major benefit of the concept as described above is that the transmission of differential rotation from the wheel axles to the sun gear 18 through the ring gear 14 and planet gear 16 generates a magnification or amplification of the rotational speed of the sun gear as compared to the difference in rotation of the wheel axles (in one example by 18xc2xd times). This magnification of rotation proportionately reduces the torque and thus the force that is required to brake or reduce the rotation of the sun gear. Whereas MPF may not be effective for directly braking the differential rotation of the wheel axles, as applied to a sun gear of a planetary gear arrangement, the braking applied by the MRF is found to be satisfactory.
Whereas the MRF fluid is the preferred fluid for the application, other materials include ERF and viscous silicon fluid.
The invention will be more fully appreciated by reference to the following detailed description having reference to the accompanying drawings.